Like many past users I need to design a traffic control system for my digital principles class.
When traffic approaches from both directions (ie, N/S and W/E) the system runs through a set routine. This part I have done.

However, when only one path is busy (only N/S or only W/E) then that path should be given a green light and the other a red, and the system needs to return to the original pattern when an oncoming car approaches.

Where I get confused is I am not sure how to switch between the two modes. If someone could get me started I would appreciate it.

 

Imagine you were standing on the street corner and had to decide between holding one direction green and the other red and rotating through your sequence. How would you decide when to switch back and forth?

Consider this question. Let's assume that the sequence has each direction getting two minutes of green. E/W currently has a green light and it is now at the two minute mark. Under what circumstances would it make sense to change the light to red (for E/W) and under what circumstances would it make sense to leave the light green?